A postmark is a postal marking made on an envelope, parcel, postcard or the like, indicating the place, date and time that the item was delivered into the care of a postal service, or sometimes indicating where and when received or in transit. Modern postmarks are often applied simultaneously with the cancellation or killer that marks postage stamps as having been used. Sometimes a postmark alone is used to cancel stamps, and the two terms are often used interchangeably. Postmarks may be applied by handstamp or machine, using methods such as rollers or inkjets, while digital postmarks are a recent innovation.

The first postmark, called the "Bishop mark", was introduced by English Postmaster General Henry Bishop in 1661 and showed only the day and month of mailing to prevent the delay of the mail by carriers.^{[user-generated source]}

In England during the latter part of the 17th century, several postmarks were devised for use with the London Penny Post, a postal system that delivered mailed items within the city of London. The postmarks bore the initial of the particular post office or handling house it was sent from along with a separate time stamp. Postage was prepaid and the postmark was applied to the mailed item by means of an inked hand-stamp. Some historians also consider these postmarks to be the world's first postage "stamps".

In the 19th century and early 1900s, it was common for letters to receive multiple postmarks, or backstamps, indicating the time, date, and location of each post office transporting or delivering the letter, and this is still occasionally true. While almost every contemporary postmark includes a location as well as a date, in 2004, New Zealand Post announced plans to eliminate the location on their postmarks and include only the date; however, information about this can be determined by a three-number code on the postmarks.

In Great Britain, the first postmark employed for the cancellation of the then new adhesive postage stamps was the Maltese Cross, so named because of its shape and appearance. This was used in conjunction with a date stamp which was applied, usually to the rear of the letter, which denoted the date of posting.

Different types of postmarks include railway post offices (RPOs) and maritime (on-board ship) postmarks. Postmarks on naval vessels during sensitive operations in wartime are sometimes "clean", showing less information than usual to prevent route of travel or other details from falling into enemy hands. Similar to this is the "censored postmark", overprinted with a black obliteration of the time and place of mailing, for similar reasons.

The Pony Express used a variety of different postmarks on the mail it carried across the Western United States. There are only 250 known examples of surviving Pony Express mail/postmarks in various collections today bearing one of more than a dozen different types of postmarks.

Hawai'i Post, a discontinued personal delivery service, once had a surfboard mail postmark, for covers that traveled by surfboard.